ABSTRACT: Our work over the last several years, indicates that cell cycle regulatory pathways are critical determinants of the response to endocrine therapy as well as targeted therapies that are frequently employed in the treatment of ER+ metastatic breast cancer. Here we will focus on the RB-tumor suppressor pathway as a central node controlling proliferation downstream of multiple pathways of relevance to therapy of ER+ breast cancer (e.g. endocrine therapy and CDK4/6 inhibitors). While RB is required for the effective cytostatic response to a range of targeted therapies employed in ER+ breast cancer, multiple pathways can contribute to ?cell cycle plasticity? and therefore represent distinct means for generating therapeutic resistance. Here we will delineate the processes underlying this form of resistance, means to elicit durable cell cycle arrest, and approaches to target resistance as observed clinically (Aim 1). Our data and newly published studies indicate that RB loss occurs in ER+ breast cancer as a means to escape from cytostatic therapies. Analysis of the RB locus in ER+ breast cancer indicates loss of one copy of 13q occurs in a significant fraction of ER+ breast cancers, suggesting that such tumors are primed for RB loss. How to subsequently treat tumors that are heterogeneous for RB or are solely RB deficient represents a significant challenge. Using drug screening and organoid approaches we have defined several regimens that are particularly effective against RB-negative tumors and could represent a general means to target ER+ tumors that progress on CDK4/6 inhibitors (Aim 2). Together these aims will interrogate means to further leverage the RB tumor suppressor for a precision approach to the treatment of ER+ breast cancer.